The research plan is designed to elucidate the renal mechanism responsible for the urinary concentrating defect observed in hypokalemic (KD) and hypercalcemic (HCa) Munich Wistar rats. Micropuncture samples will be collected from late proximal, early and late distal segments of superficial nephrons, and descending limbs and collecting ducts within the papilla to evaluate the follgwing: osmotic equilibration across ADH sensitive nephron segments, loop and collecting duct transport of NaCl, intrarenal urea recycling, and the effects of acute urea infusion on the concentrating defect. Papillary plasma flow will be measured in a separate group of experiments. In addition, to examine more precisely the renal response to ADH, papillary collecting duct permeability to urea and water, and osmotic flow of water in the presence and absence of ADH will be measured in vivo using microperfusion technique.